Vibrations are commonly experienced in rotating machine components. In many cases, these vibrations can lead to product irregularities during manufacture, premature wear and failure of machine components, and unbearable environments. At the design level, the components are often sized so that their natural frequencies are well above the frequencies expected from vibration sources. To further reduce or prevent significant vibrations, balancing of individual components is often performed. Quite often, these preventative measures are insufficient at reducing the vibration levels to tolerable levels. Other techniques must be applied after the fact. In some corrective measures, damping is added to the components. With rotating machinery, the operational speeds are often adjusted to reduce certain frequencies of vibrations. In many cases, such measures are insufficient at controlling vibration, so more active techniques are required.
Paper machine rolls have specific challenges with vibrations. The rolls turn at certain rotational speeds, which will produce a level of vibration at the same frequency for any level of imbalance. Nipped rolls have additional challenges as follows:                1) Vibrations may occur at frequencies related to common multiples of roll diameters or felt length. For example, if one roll were three-fifths the diameter of its mating roll, there could be 15 locations on the larger roll where the same dot of the smaller roll touches. The larger roll could then vibrate at frequencies 15 times that of its rotational frequency. Felt seams of press felts often employed in paper machines have been blamed for inducing these types of vibrations and wear. This effect is sometimes called barring.        2) Other sources of wear can increase the levels of vibrations. If the nip is considered as a stiff spring, and the roll bodies as masses, this spring-mass system will typically vibrate at its natural frequency. If a roll vibrates at a frequency that is 15 times greater than its rotational frequency, fifteen worn or barred regions would be generated.        3) A paper sheet that is traveling into the nip may itself have irregularities, such as cyclical density, stiffness, or thickness variations. As such a sheet passes through the nip, the nip pressures will have a cyclical variation and vibrations may result. This source is often reported in calender stacks.        4) The roll could have beam bending vibrations.        5) Coatings on the rolls can be eccentric to the core or with the journal which causes vibrations at the same frequency as the roll rotation. Even when such a roll is dynamically balanced, the roll cover thickness variation causes a cyclical variation in the nip pressure. For example, if the cover is thickest at zero degrees and thinnest at 180 degrees, the rolls will deflect more at zero degrees than at 180 degrees. A vibration will result in addition to the pressure variation seen by the paper sheet.        
Common reactions to excessive vibration levels include adjusting the operational speed, resurfacing the rolls, and changing the roll cover material. Changes to the operational speed are typically undesirable, since the other sections of the paper machine are optimized for a different speed; also, slower speeds reduce productivity and higher speeds may reduce quality. Resurfacing the rolls by regrinding the finish and final diameters usually involves substantial downtime and resurfacing costs. Roll covers having increased damping properties have also been developed to decrease the levels of vibrations; however, changing to a different cover material involves significant downtime to replace the rolls, and significant time and financial costs to replace the covering.
Present monitoring techniques usually involve placing vibration sensors at the bearing mounts and near the ends of the rolls. These sensors can pick up the major effects of the vibrations, but can not always pinpoint the source. It may be desirable to provide a vibration detection system that can generate more and better information about the vibrations of the roll.